This invention relates to a polymerization process for propylene. In particular, the invention relates to premixing a magnesium halide-supported Ziegler-Natta catalyst with a portion of an organoaluminum cocatalyst prior to polymerization.
Ziegler-Natta catalysts, by which we mean a transition metal compound that incorporates a Group 4-8 transition metal, preferably a Group 4-6 transition metal, and one or more ligands that satisfy the valence of the metal are known to effectively promote the high yield polymerization of olefins possessing commercially desirable characteristics. However, the use of conventional Ziegler-Natta catalysts is subject to important limitations. Thus, new and improved catalysts are continually being sought and developed.
It is known that Ziegler-Natta catalysts can be supported on magnesium halides. U.S. Pat. No. 4,124,532 discloses the polymerization of ethylene and propylene with high activity catalysts. These catalysts comprise complexes which may contain magnesium and titanium. These complexes are prepared by reacting the halide MX2 (where M may be Mg) with a compound M""Y (where M"" may be Ti and Y is one or more halogens or an organic radicals) in an electron donor compound. These complexes are then isolated by either crystallization, by evaporation of the solvent or by precipitation. Polymerization is carried out with these complexes and an alkyl aluminum compound.
U.S. Pat. No. 4,173,547 to Graff discloses contacting a carrier with an organoaluminum halide and a dialkylmagnesium compound and then contacting the treated carrier with an organotitanium compound.
U.S. Pat. No. 4,302,565 to Goeke et al. and U.S. Pat. No. 4,302,566 to Karol et al. disclose catalyst systems for producing ethylene copolymers provided by a xe2x80x9cprecursor compositionxe2x80x9d which is the solid reaction product of magnesium chloride and titanium tetrachloride. This xe2x80x9cprecursor compositionxe2x80x9d is activated with an organoaluminum compound. They set forth a continuous process for ethylene copolymer production employing a gas phase fluidized bed vertical tubular reactor. A catalyst system is provided by a xe2x80x9cprecursor compositionxe2x80x9d which is the solid reaction product of magnesium chloride and titanium tetrachloride. This xe2x80x9cprecursor compositionxe2x80x9d is activated with an organoaluminum compound. Two methods are disclosed. One is by dry blending which has the disadvantage of handling a pyrophoric solid. The other activates in a hydrocarbon slurry, the hydrocarbon solvent is removed by drying and the partially activated precursor composition is fed to the polymerization reactor where the activation is completed with additional activator compound which can be the same or a different compound. In both methods, a catalyst preparation procedure separate from the polymerization is needed. This results in extra cost and complexity.
U.S. Pat. No. 4,732,882 to Allen et al. teaches a supported organomagnesium compound which is contacted with a transition metal compound and then with trimethylaluminum. Allen teaches that trimethylaluminum is superior to other alkyl aluminum compounds, but does not recognize the benefit of premixing a portion of the alkyl aluminum. In fact, Allen teaches that the method of combining trimethylaluminum with the catalyst precursor is not critical.
U.S. Pat. No. 4,833,111 to Nowlin contacts an alcohol with a slurry of a dialkylmagnesium compound and a support and then treats the resultant slurry with a transition metal compound. This resultant slurry is contacted with a halogenated alkylaluminum compound and then that product is activated with trimethylaluminum. While Nowlin teaches improved activity due to the pretreatment with the halogenated alkyl aluminum compound he does this by first reacting the slurry of the transition metal compound with solvent, then removing the solvent under reduced pressure and then treating the catalyst precursor with trimethylaluminum. Nowlin""s process is complicated, involves several steps, and requires specific and different aluminum compounds.
U.S. Pat. No. 6,187,866 provides for the in-situ blending of polymers by contacting ethylene and one or more comonomers in two or more fluidized bed reactors connected in series under polymerization conditions with a catalyst system. The catalyst system is based upon a magnesium/titanium based precursor containing an electron donor which is treated with a precursor activator to influence the melt flow ratio of the polymer blend and then with a hydrocarbyl aluminum cocatalyst to complete the activation. By this process, they control the melt flow ratio of the blend or the bulk density of the blend. To activate the catalyst slurry, they treat with a first alkyl aluminum compound for 1-4 hours and then with a second alkyl aluminum compound for another 1-4 hours followed by complete activation in the polymerization reactor. The process is complicated and limited to the in situ blending of polymers.
Despite the importance of olefin polymerizations and the considerable research that has been done on various catalyst systems, there remains a need to improve the activity of the catalyst. This can be important from a cost view since the catalyst is typically one of the more costly ingredients. Similarly, the equipment for catalyst handling can add to the cost. Any improvement in catalyst activity decreases these costs. However, even more important is that the residual metal in the polymer is reduced. High levels of residual metal can have a deleterious effect on polymer properties such as color and aging. It is therefore important to keep the residual metals as low as possible. Any improvement in catalyst activity lowers the residual metals in the polymer.
The invention is a process for making polypropylene. A magnesium halide-supported Ziegler-Natta catalyst is premixed with 1 to 10% of an organoaluminum cocatalyst. This premix is then added to a heated mixture containing propylene and 90 to 99% of the organoaluminum cocatalyst. Temperature is maintained for the reaction mixture to produce polypropylene.
The process of the invention is easy to practice and affords enhanced catalyst activity. Since the catalyst is not removed from the final polymer, an increase in activity results in a polymer with lower residual metals. The process is robust and gives an improvement in activity for copolymers of propylene as well as the homopolymer. It is effective in the presence of molecular weight regulators and modifiers which can be used in propylene polymerizations.
In the first step of the process of the invention, propylene is mixed with from about 90 to 99% of the organoaluminum cocatalyst.
The organoaluminum cocatalyst is an alkyl aluminum or an alkyl aluminum halide. Preferred alkyl aluminums include trialkyl or triaryl aluminum compounds, which preferably have the formula AIR3 where each R is a C1-C30 hydrocarbyl. Particularly preferred alkyl aluminums are trimethylaluminum, triethylaluminum, tri-n-propylaluminum triisopropyl-aluminum, tri-n-butylaluminum, triisobutylaluminum, and tri-n-hexyl-aluminum. Suitable alkyl aluminum halides include dialkyl aluminum halide and alkyl aluminum dihalide compounds, which preferably have the formula AIR2X or AIRX2 where X is Cl, Br, or I.
Exemplary alkyl aluminum halides are dimethylaluminum chloride, methylaluminum dichloride, diethylaluminum chloride, ethylaluminum dichloride, diisobutylaluminum chloride, isobutylaluminum dichloride, methylaluminum sesquichloride, ethylaluminum sesquichloride, and isobutylaluminum sesquichloride.
Preferably, organosilane modifiers are used in the polymerization. These help to maintain and control the stereoregularity of the polymer. They can also offer certain improvements such as an improved sensitivity to hydrogen as a means of controlling molecular weight. Preferred organosilane modifiers are alkyl alkoxysilanes which have the formula RySi(OR)4xe2x88x92y where R is as previously described and y is an integer from 1 to 3. More preferably, the organosilane modifiers are dialkyl dialkoxy silanes. Exemplary dialkyl dialkoxysilanes are diisopropyldimethoxysilane, isobutyl-isopropyldimethoxysilane, cyclohexylmethyidimethoxysilane, dimethyldimethoxysilane, dimethyldiethoxysilane and dicyclopentyldimethoxysilane.
If a copolymer of propylene is desired, an olefin comonomer is added with the propylene. Suitable olefin comonomers are C2-C20 xcex1-olefins, such as ethylene, 1-butene, 1-hexene, 1-octene and mixtures thereof. Preferred olefin comonomers are ethylene, and mixtures thereof with xcex1-olefins such as 1-butene, 1-hexene and 1-octene.
Optionally, hydrogen is added to regulate the molecular weight. Generally, the greater the level of hydrogen added, the lower the molecular weight.
In the second step, the propylene/cocatalyst mixture is heated to at least about 50xc2x0 C. Preferably, the propylene/cocatalyst mixture is heated to about 60xc2x0 C. to about 90xc2x0 C.
In the third step, the remaining 1 to 10% of the organoaluminum cocatalyst is premixed with a magnesium halide-supported Ziegler-Natta catalyst and this premix is added to the heated propylene/cocatalyst mixture. The cocatalyst is an organoaluminum cocatalyst as described above for the first step. While it is possible to use one organoaluminum cocatalyst for step 1 and a different organoaluminum cocatalyst for premixing with the Ziegler-Natta catalyst, typically, for simplicity, the same organoaluminum cocatalyst is used in both steps. The premixture time need not be an extended time and can be just a matter of seconds. For a continuous process, the premixture can be done in the process stream by continuous addition of organoaluminum cocatalyst to the process stream containing the magnesium halide-supported Ziegler-Natta catalyst feed prior to entry into the reactor. In other words, the premixing can be done in situ as the combined streams are flowing to the reactor. This simplifies the process and avoids any potential issues with shelf life of the alkylated catalyst.
Catalysts useful in this invention are Ziegler-Natta catalysts supported on magnesium chloride, magnesium bromide or magnesium iodide. Preferably, anhydrous magnesium chloride is used. The Ziegler-Natta catalyst is admixed with the magnesium halide. By xe2x80x9cZiegler-Natta catalyst,xe2x80x9d we mean a transition metal compound that incorporates a Group 4-8 transition metal, preferably a Group 4-6 transition metal, and one or more ligands that satisfy the valence of the metal. The ligands are preferably halide, alkoxy, hydroxy, oxo, alkyl, and combinations thereof. Preferred Ziegler-Natta catalysts incorporate Ti, V, or Cr, most preferably Ti. Preferred Ziegler-Natta catalysts also have high thermal stability. They include titanium halides, titanium alkoxides, vanadium halides, and mixtures thereof, especially, TiCl3, TiCl4, mixtures of VOCl3 with TiCl4, and mixtures of VCl4 with TiCl4. Other suitable Ziegler-Natta catalysts appear in U.S. Pat. No. 4,483,938, the teachings of which are incorporated herein by reference, and in Eur. Pat. 222,504. A particularly preferable admixture is TiCl4 supported on MgCl2.
Preferably, the magnesium halide-supported Ziegler-Natta catalysts also contain a donor. These donors are Lewis bases. Suitable electron donor compounds include alkyl esters of aliphatic and aromatic carboxylic acids, aliphatic ethers, cyclic ethers and aliphatic ketones. The use of electron donors appears in U.S. Pat. No. 4,302,565, the teachings of which are incorporated herein by reference. Preferred donors are alkyl esters of aromatic carboxylic acids.
Optionally, the magnesium halide-supported catalysts are further supported on an oxide such as silica or alumina to provide a high surface area, inert support upon which to distribute the active ingredients and obtain good particle morphology.
While not wanting to be bound by theory, we believe that in this third step of the process of the invention, the remaining 1 to 10% of the organoaluminum cocatalyst prealkylates the catalyst under controlled conditions, thereby improving the catalyst activity. Preferably, this premixing is done at or below 40xc2x0 C., more preferably between about xe2x88x9220xc2x0 C. to about 20xc2x0 C.
Optionally, the premixing is done in the presence of a portion of the propylene to be used or in an inert hydrocarbon solvent to facilitate mixing and handling.
The premix of the remaining 1 to 10% of the organoaluminum cocatalyst and supported Ziegler-Natta catalyst is added to the reactor mixture, which has been preheated to a temperature suitable for polymerization. This temperature is at least about 50xc2x0 C. and preferably from about 60xc2x0 C. to about 90xc2x0 C. The temperature of the reaction mixture is maintained to produce polypropylene. Preferably, the polymerization is performed under pressure. During the polymerization, the pressure can be maintained by continuously feeding propylene, comonomer, hydrogen or a mixture of these depending upon the desired polymer properties.
A variety of types of polymerization processes can be used. Preferably, the process is practiced in the liquid phase, which can include slurry, suspension or bulk processes or a combination of these. High-pressure fluid phase or gas phase techniques can also be used. The process of the invention is particularly valuable for solution and slurry processes.